Polymorphisms in the RET proto-oncogene and the phenotypic presentation of familial medullary thyroid carcinoma

Molecular Basis and Natural History of Medullary Thyroid Cancer: It is (Almost) All in the RET

Medullary thyroid cancer (MTC) is a rare disease, which can be either sporadic (roughly 75% of cases) or genetically determined (multiple endocrine neoplasia type 2, due to REarranged during Transfection RET germline mutations, 25% of cases). Interestingly, RET pathogenic variants (mainly M918T) have also been reported in aggressive forms of sporadic MTC, suggesting the importance of RET signalling pathways in the pathogenesis of MTC. The initial theory of RET codon-related MTC aggressiveness has been recently questioned by studies suggesting that this would only define the age at disease onset rather than the aggressiveness of MTC. Other factors might however impact the natural history of the disease, such as RET polymorphisms, epigenetic factors, environmental factors, MET (mesenchymal-epithelial transition) alterations, or even other genetic alterations such as RAS family (HRAS, KRAS, NRAS) genetic alterations. This review will detail the molecular bases of MTC, focusing on RET pathways, and the potential mechanisms that explain the phenotypic intra- and interfamilial heterogeneity.

Variability in Medullary Thyroid Carcinoma in RET L790F Carriers: A Case Comparison Study of Index Patients

Background: Previous studies have suggested that the variability in age of onset and aggressiveness of medullary thyroid carcinoma (MTC) in patients with multiple endocrine neoplasia type 2A (MEN 2A) carrying the same REarranged during Transfection (RET) mutation may be caused by additional RET germline variants or somatic variants. Methods: This study was a retrospective case comparison study of all MEN 2A index patients (n = 2) with the RET L790F germline mutation in Denmark. Whole blood and MTC tissue were analyzed for RET germline variants and other somatic variants (>500), respectively. Results: Patient 1 presented with MTC (T1aN1bM0) at age 14 years, while patient 2 presented with MTC (T1bN0M0) at age 70 years. No germline RET germline variants nor other variants were found to explain this MTC variability. Conclusions: We could not confirm the previously reported finding of a somatic RET variant as likely responsible for the early onset and aggressiveness of MTC in a RET germline mutation carrier. Also, we found no RET germline variants that could explain the MTC variability among our index patients. We did, however, identify a somatic FLT3 R387Q variant with an unknown potential as genetic modifier. Further large-scale studies are needed to investigate genetic modifiers in RET L790F carriers.

Genetic diagnosis of a Chinese multiple endocrine neoplasia type 2A family through whole genome sequencing

Approximately 98% of patients with multiple endocrine neoplasia type 2A (MEN 2A) have an identifiable RET mutation. Prophylactic or early total thyroidectomy or pheochromocytoma/parathyroid removal in patients can be preventative or curative and has become standard management. The general strategy for RET screening on family members at risk is to sequence the most commonly affected exons and, if negative, to extend sequencing to additional exons. However, different families with MEN 2A due to the same RET mutation often have significant variability in the clinical exhibition of disease and aggressiveness of the MTC, which implies additional genetic loci exsit beyond RET coding region. Whole genome sequencing (WGS) greatly expands the breadth of screening from genes associated with a particular disease to the whole genome and, potentially, all the information that the genome contains about diseases or traits. This is presumably due to additive effect of disease modifying factors. In this study, we performed WGS on a typical Chinese MEN 2A proband and identified the pathogenic RET p.C634R mutation. We also identified several neutral variants within RET and pheochromocytoma-related genes. Moreover, we found several interesting structural variants including genetic deletions (RSPO1, OVCH2 and AP3S1, etc.) and fusion transcripts (FSIP1-BAZ2A, etc.).

Role of RET genetic variants in MEN2-associated pheochromocytoma

BACKGROUND

RET polymorphisms have been involved in the clinical presentation and prognosis of multiple endocrine neoplasia type 2 (MEN2)-associated medullary thyroid carcinoma.

OBJECTIVE

To investigate the effect of RET variants on the penetrance of pheochromocytoma (PHEO) in MEN2 patients.

METHODS

The RET variants L769L, S836S, and G691S/S904S were evaluated in a cohort of 153 MEN2 patients attending a tertiary teaching hospital. A comparison of RET variant frequencies between patients with and without PHEO was performed. Kaplan-Meier curves and Cox regression analysis were used to estimate the effect of RET variants on the age-dependent penetrance.

RESULTS

A total of 48 (31.4%) patients presented with MEN2-associated PHEOs. The mean age at diagnosis was 35.5±13.4 years, 60.4% of patients were women, and 92.8% had RET mutations at codon 634. The frequencies of RET polymorphisms were as follows: 20.1% L769L, 4.75% S836S, and 17.3% S904S/G691S. We did not observe any association between the frequencies of L769L, S836S, or S904S/G691S variants and PHEO development (all P>0.05). However, individuals carrying two RET polymorphic alleles had an increased estimated risk of PHEO (2.63; 95% CI, 1.4-5.0; P=0.004) and were younger at diagnosis when compared with those with one or no polymorphism (29.6±6.3 and 39.3±14.4 years respectively; P=0.006). Accordingly, additional analysis using Cox proportional hazard models demonstrated that the presence of two RET variants was associated with an increased risk for early PHEO development (hazard ratio, 5.99 (95% CI, 2.24-16.03); P<0.001).

CONCLUSIONS

RET polymorphic alleles have an additive effect on the estimated risk of age-related PHEO penetrance in MEN2 patients.

Comprehensive analysis of RET gene should be performed in patients with multiple endocrine neoplasia type 2 (MEN 2) syndrome and no apparent genotype-phenotype correlation: an appraisal of p.Y791F and p.C634Y RET mutations in five unrelated Brazilian families

BACKGROUND

We previously identified a four-generation family with medullary thyroid cancer (MTC) and a germline p.Y791F RET mutation whose cancer lacked a strong genotype-phenotype correlation. The entire gene coding region of the RET gene should be sequenced when genotype-phenotype discrepancies are observed in patients with multiple endocrine neoplasia type 2 (MEN 2), even if a RET hotspot mutation has been identified.

METHODS

A new genetic test was performed in the index case of this family with the p.Y791F RET germline mutation. The entire coding region of the RET gene was investigated by direct sequencing of PCR products. Once a mutation was identified, the target exon was sequenced in all at-risk relatives.

RESULTS

An additional p.C634Y germline mutation in the RET gene was identified in the reported family. The double mutation occurred in cis and segregated with the phenotype. Through the Brazilian Genetic Screening Program developed at our institution, we additionally report the combination of these two mutations (p.C634Y/p.Y791F) in the RET gene in four other unrelated families. The overall penetrance of MTC and pheochromocytoma in patients with the p.C634Y/p.Y791F mutations was 79% and 13%, respectively.

CONCLUSION

Our data emphasises that a comprehensive analysis of the RET gene may reveal multiple germline mutations in MEN 2 patients who exhibit an atypical clinical course of the disease.

The optimal range of RET mutations to be tested: European comments to the guidelines of the American Thyroid Association

In the 9^th ETA-CRN Meeting (September 2009, Lisbon) some recommendations from the American Thyroid Association (ATA) guidelines for the management of medullary thyroid cancer (MTC) were discussed by an European Panel of Experts (EPE). Among the 12 ATA recommendations related to hereditary MTC and to the optimal range of RET mutations to be tested (recommendations 1-5 and 9-15), 7 were shared and 5 were not shared by the EPE. In the present paper, the related comments and suggestions will be reported and discussed.

Single nucleotide polymorphism rs17849071 G/T in the PIK3CA gene is inversely associated with follicular thyroid cancer and PIK3CA amplification

The proto-oncogene PIK3CA has been well studied for its activating mutations and genomic amplifications but not single nucleotide polymorphism (SNP) in thyroid cancer. We investigated SNP rs17849071 (minor allele G and major allele T) in PIK3CA in thyroid tumors in 503 subjects by PCR and sequencing of a region of intron 9 carrying this SNP. This SNP was found in both normal and thyroid tumor tissues as well as in different generations of a studied family, confirming it to be a germline genetic event in thyroid tumor patients. In comparison with normal subjects, a dramatically lower prevalence of the heterozygous genotype G/T at rs17849071 was found in patients with follicular thyroid cancer (FTC). Specifically, rs17849071G/T was found in 15% (18/117) normal subjects vs. 1.3% (1/77) FTC patients, with an odds ratio of 0.07 (95% CI 0.01–0.55; P = 0.001). This represents a 93% risk reduction for FTC with this SNP. In contrast, no difference was seen with benign thyroid neoplasms in which the prevalence of rs17849071G/T was 13.1% (17/130), with an odds ratio of 0.83 (95% CI 0.40–1.69; P = 0.72). There was a trend of lower prevalences of rs17849071G/T and odds ratio in other types of thyroid cancer without statistical significance. We also found an interesting inverse relationship of rs17849071G/T with PIK3CA amplification. With copy number ≥4 defined as copy gain, 2.9% (1/34) rs17849071G/T vs. 19.0% (67/352) rs17849071T/T cases displayed PIK3CA amplification (P = 0.01). Conversely, 1.5% (1/68) cases with PIK3CA amplification vs. 10.4% (33/318) cases without PIK3CA amplification harbored rs17849071G/T (P = 0.01). This provides an explanation for the reciprocal relationship of rs17849071G/T with FTC, since PIK3CA amplification is an important oncogenic mechanism in thyroid cancer, particularly FTC. Thus, the present study uncovers an interesting phenomenon that rs17849071G/T is protective against FTC possibly through preventing PIK3CA amplifications.

Molecular basis of medullary thyroid carcinoma: the role of RET polymorphisms

Medullary thyroid carcinoma is a rare malignant tumor originating in parafollicular C cells. It accounts for 5 to 8% of all thyroid cancers. MTC develops in either sporadic (75%) or hereditary form (25%). Genetic and molecular studies have demonstrated the involvement of the RET proto-oncogene in hereditary MTC and, less often, in its sporadic form. Although a strong genotype-phenotype correlation has been described, wide clinical heterogeneity is observed among families with the same RET mutation or even in carriers of the same kindred. In recent years, several single nucleotide polymorphisms of the RET gene have been described in the general population as well as in patients with MTC. Some studies have reported associations between the presence of polymorphisms and development or progression of MTC. Nonetheless, other studies failed to demonstrate any effect of the RET variants. Differences in the genetic background of distinct populations or methodological approaches have been suggested as potential reasons for the conflicting results. Here, we review current knowledge concerning the molecular pathogenesis of sporadic and hereditary MTC. In particular, we analyze the role of RET polymorphisms in the clinical presentation and prognosis of MTC based on the current literature.

RET-protooncogene variants in patients with sporadic neoplasms of the digestive tract and the central nervous system

PURPOSE

The RET protooncogene plays a crucial role in neural crest development; accordingly, mutations of RET cause MEN2A and familial medullary thyroid carcinoma, while the expression deregulation of RET is involved in the pathophysiology of glioblastoma multiforme (GBM) and pancreatic cancer (PDAC). The aim of this study was to evaluate if germline variants of the RET protooncogene are associated with GBM, pancreatic cancer and gastric cancer (GC).

METHODS

Genomic DNA from peripheral blood was isolated from 100 patients with GBM, 65 patients with GC and 54 patients with PDAC. The coding sequence of RET promoter, exon 2 and exon 13 was amplified. Sequence variations at -5 and -1 in the promotor and in exon 2 were determined through a LightCycler assay, and analysis of exon 13 was carried out by genomic sequencing.

RESULTS

There was no significant association of the RET-promoter or exon 2 genotypes with the phenotype in the different populations, although there was an increase of the GG genotype of the -5G>A variant in all cancers compared to controls. Sequencing of exon 13 identified mutation c.2372A>T in codon 791 (Y791F) in heterozygous state in one of 100 GBM patients, in two of 65 patients with gastric cancer, in two of 54 PDAC patients and in none of the controls.

CONCLUSIONS

Although our data did not reach significance in our small cohorts, we cannot rule out the involvement of the -5G promoter allele and the c.2372A>T mutation in the development of the aforementioned tumours.

Evaluation of RET polymorphisms in a six-generation family with G533C RET mutation: specific RET variants may modulate age at onset and clinical presentation

CONTEXT

We previously described a six-generation family with G533C RET mutation and medullary thyroid carcinoma, in the largest family reported do date. Of particular interest, phenotype variability regarding the age of onset and clinical presentation of the disease, was observed.

OBJECTIVE

We evaluate whether single SNPs within RET oncogene or haplotype comprising the RET variants (defined by Haploview) could predispose to early development of MTC in this family and influence the clinical manifestation.

DESIGN

Eight SNPs were selected based on their previous association with the clinical course of hereditary or sporadic MTC, in particular promoting an early onset of disease. The variants were initially tested in 77 G533C-carriers and 100 controls using either PCR-direct sequencing or PCR-RFLP. Association between a SNP or haplotype and age at diagnosis or presence of lymph node metastasis was tested in 34 G533C-carries with MTC. Different bioinformatic tools were used to evaluate the potential effects on RNA splicing.

RESULTS

An association was found between IVS1-126G > T and age at diagnosis. The variant [IVS8 +82A > G; 85-86 insC] was associated with the presence of lymph node metastases at diagnosis. In silico analysis suggested that this variant may induce abnormal splicing. This in silico analysis predicted that the [IVS8 +82A > G; 85-86 insC] could alter the splicing by disrupting and/or creating exonic splicing enhancer motifs.

CONCLUSIONS

We here identified two RET variants that were associated with phenotype variability in G533C-carriers, which highlights the fact that the modifier effect of a variant might depend on the type of mutation.

Familial medullary thyroid carcinoma associated with cutaneous lichen amyloidosis

BACKGROUND

This is a report of a patient with a novel genotype-phenotype relationship of a c804 mutation of the RET proto-oncogene manifesting as medullary thyroid carcinoma (MTC) and cutaneous lichen amyloidosis (CLA).

SUMMARY

Clinical data were obtained for patient appearance and laboratory results. Analyzed were histopathology of the skin lesion and thyroid gland, genetic mutation, and family pedigree. Skin histology and histochemistry were consistent with CLA. Serum calcitonin levels were moderately elevated. Thyroid histology demonstrated a 4 mm focus of MTC. Measurements of serum parathormone, calcium, and plasma metanephrines were normal. DNA analysis demonstrated a mutation in codon 804 of the RET proto-oncogene resulting in a Valine to Methionine (V804M) substitution. Genetic testing in two siblings revealed the same mutation.

CONCLUSIONS

This is the first description of a patient with CLA not associated with a mutation in codon 634. The patient is one of the few with a V804M mutation in whom the clinical expression did not fully conform to the definition of familial MTC.

[Hereditary thyroid cancer]

Hereditary thyroid carcinomas are present in about 5% of differentiated (DTC) and 25% of medullary thyroid carcinomas (MTC). They are part of a multiorgan tumour syndrome (e. g. FAP Gardner's syndrome with DTC and MEN 2 syndrome with MTC) or confined to the thyroid gland. Hereditary thyroid carcinomas typically show multifocal growth and occur in young patients. Due to germ cell mutations as the underlying cause of disease, partial thyroidectomies that may be justified in early sporadic carcinomas are not indicated in this type of tumours. In the case of hereditary DTC, the genetic basis of the disease has been demonstrated only in syndromatic tumour variants. In most nonsyndromatic cases, specific genetic alterations have not yet been identified. In both types of hereditary DTC, prophylactic thyroidectomy is not warranted due to the favourable prognosis of tumours that do not differ from sporadic ones. Point mutations of the RET proto-oncogene have been known for 15 years to be the genetic basis of hereditary MTC. Recently several new mutations were discovered; however, final conclusions regarding their clinical significance are not possible at present. Basically it has been shown that the clinical aggressivity of tumour development follows a genotype-phenotype correlation (risk groups 1-3). However, in mutations of all risk classes there exists a wide spectrum of different stages of hereditary C-cell disease in individual risk groups. Regarding time and extent of prophylactic thyroidectomy (without or with lymph node dissection) a combined molecular-biochemical concept including the use of pentagastrin-stimulated calcitonin values is therefore recommended.

RET genotypes in sporadic medullary thyroid cancer: studies in a large Italian series

BACKGROUND

Highly discrepant data about the different distribution of RET germline single nucleotide polymorphisms (SNPs) among patients with sporadic medullary thyroid cancer (sMTC) and controls are available.

DESIGN AND PATIENTS

In the present case-control study, a wide panel of seven RET SNPs has been tested in the largest sMTC series and in a matched control group.

RESULTS

None of the investigated polymorphisms show a significantly different distribution in patients with sMTC when compared to controls. Twenty haplotypes and 57 genotypes were generated, and their association with the disease and with the clinical features were statistically evaluated. Interestingly, 14 genotypes were found to be unique to sMTC patients and 25 to controls. Two haplotypes and three genotypes, all including the intronic variants IVS1-126 and IVS14-24, were significantly associated with sMTC patients and with a higher tumour aggression. The functional activity of the only nonsynonymous RET variant (c.2071C > A, G691S) was tested for the first time. Interestingly, Western blot analyses showed that the fraction of Ret9-G691S protein located at the plasma membrane level was overrepresented when compared to Ret9-WT, suggesting facilitated targeting at the cell membrane for this variant. However, no transforming activity was shown in a focus formation assay on cells carrying the Ret9-G691S, against a possible oncogenic role of G691S variant.

CONCLUSIONS

RET genotypes including two intronic RET variants were associated with the risk of developing sMTC and to more aggressive behaviour. Further studies are warranted to elucidate whether these RET genotypes are in linkage disequilibrium with another susceptibility gene or whether these variants could play a role in the genesis of sMTC per se.

Hypereosinophilia as the presentation of metastatic medullary thyroid carcinoma: a remarkable event

Hypereosinophilic syndrome is a rare disorder characterized by persistent eosinophilia combined with organ system dysfunction. This report is of a 37-year-old man who had eosinophilia, periodic fever, weight loss, and generalized bone pain. A small nodule in left lobe of thyroid was detected in his physical examination. The patient underwent surgery and histopathological study, which confirmed that the patient had medullary carcinoma of thyroid. The patient died 6 months after the surgery. Eosinophilia is more commonly seen with benign conditions. However, less frequent but malignant etiologies always should be contemplated.

Asymptomatic bilateral adrenal pheochromocytoma in a patient with a germline V804M mutation in the RET proto-oncogene

A diagnosis of bilateral pheochromocytoma warrants exclusion of hereditary pheochromocytoma.

OBJECTIVE

To describe the first case of a bilateral pheochromocytoma associated with V804M mutation in the RET proto-oncogene.

PATIENTS

The index case was a 54-year-old man with bilateral adrenal masses discovered during a CT scan performed for other reasons.

MEASUREMENTS

Genetic analysis included exons 8-11 and 13-17 in the RET proto-oncogene, all four exons and flanking intronic regions in the SDHD gene, all eight exons and flanking intronic regions in the SDHB, and all three exons in the VHL gene.

RESULTS

Investigations revealed elevated urinary metanephrines (32.3 micromol/day), and laparoscopic bilateral adrenalectomy confirmed bilateral pheochromocytomas. A heterozygous V804M mutation in exon 14 of the RET was found in the index case and in four relatives. Total thyroidectomy, performed in four of five affected members in this kindred, disclosed a medullary thyroid carcinoma in the index case and in a 50-year-old woman, and nodular C-cell hyperplasia in the other two subjects.

CONCLUSIONS

This clinical case suggests that individuals carrying the germline V804M mutation should be screened annually for the presence of pheochromocytoma.

Polimorfismos genéticos: implicações na patogênese do carcinoma medular de tireóide

O carcinoma medular de tireóide (CMT) é uma neoplasia maligna rara, ocorrendo na forma esporádica ou hereditária. Mutações germinativas no proto-oncogene RET são responsáveis pelo CMT hereditário. No entanto, a maioria dos casos de CMT ocorre em indivíduos sem história familiar, na qual a patogênese da doença ainda é pouco compreendida. Os polimorfismos do gene RET são descritos na população geral assim como em pacientes com CMT. Embora estas variações alélicas aparentemente não confiram qualquer atividade transformadora no receptor RET, estudos sugerem que essas alterações genéticas podem modificar a suscetibilidade à doença e o fenótipo clínico em pacientes com CMT esporádico ou hereditário. Uma maior freqüência dos polimorfismos localizados nos exons 11 (G691S), 13 (L769L), 14 (S836S) e 15 (S904) é descrita em pacientes com CMT provenientes de países americanos e europeus. Na presente revisão, analisamos criticamente os resultados obtidos nos diferentes estudos e descrevemos a freqüência dos polimorfismos do RET em pacientes brasileiros com CMT esporádico.

Familial medullary carcinoma prevention, risk evaluation, and RET in children of families with MEN2

The ability to predict the risk of MEN2 and medullary thyroid carcinoma (MTC) by genetic RET proto-oncogene analysis has provided an essential tool in identifying patients in whom thyroid cancer can be prevented by prophylactic thyroidectomy but emphasizes the need for clear policy guidelines. Children of families with RET cysteine mutations (exons 10, 11, 13, and 16) may develop early metastatic tumours and require prophylactic thyroidectomy. The 918 mutation associated with MEN2B is associated with early aggressive behaviour and distant metastatic spread. This has led to active screening of affected families underlining the need for specific intervention strategies.

AIM

To evaluate the risk to children of families with MEN2 and to assess the risk and determine the treatment.

METHODS

Twenty-five patients from 10 families with MEN2 phenotypes were screened for RET mutations. Polymerase chain reaction amplification was performed on all 21 exons of the RET proto-oncogene, followed by heteroduplex single-strand conformation polymorphism (HEX-SSCP) analysis. Polymerase chain reaction products demonstrating variation in the HEX-SSCP gels were subjected to automated DNA sequencing analysis.

RESULTS

Eleven significant RET mutations were detected in affected families. Eight index cases received initial thyroidectomy for established MTC (plus 2 advised). In the family members screened, 3 prophylactic thyroidectomies (2 with early MTC) were performed and a further 2 recommended. An exon 10 C620W missense mutation (the "Janus" gene) was detected in a patient with Hirschsprung's disease plus 1 family member.

CONCLUSION

RET analysis of MEN has revolutionized the management of children of families with MEN2 and allowed surgical prediction and prophylaxis to take place. The presence of an exon 10 C620W mutation in association with Hirschsprung's disease was difficult to assess. We suggest possible guidelines for management of families with MTC and the role of genetic testing in their evaluation.

Significance of the RET proto-oncogene polymorphisms in Turkish sporadic medullary thyroid carcinoma patients

Several single nucleotide polymorphisms (SNP) of the RET gene have been identified in medullary thyroid carcinoma (MTC) patients as well as in the general population. However, the relevance of SNP for MTC patients is still controversial, whether these allelic variants play other interacting, predisposing or modifying roles in clinical behavior of MTC. The aim of this work is to elaborate allelic frequencies of the RET proto-oncogene polymorphisms in Turkish sporadic MTC patients and to demonstrate if there is an association between SNP and the clinical disease features, specifically the age at onset of MTC and lymph node involvement at diagnosis. We analyzed the allelic frequencies of SNP of the exon 11, 13, 14 and 15 of the RET proto-oncogene in blood samples from 50 sporadic MTC patients, using the polymerase chain reaction methodology followed by DNA sequencing. The observed allelic frequencies were 24% for G691S polymorphism in exon 11, 29% for L769L polymorphism in exon 13, 5% for S836S polymorphism in exon 14, and 26% for S904S polymorphism in exon 15. These frequencies are similar to those reported in other countries. We did not observe any significant association of all four SNP with the age at onset of MTC. Our results indicate a possible association between the presence of lymph node involvement at the time of diagnosis (extent of disease) and L769L or S836S polymorphism. However, it is not possible to draw definitive conclusions that these two polymorphisms play a significant role in clinical behavior of MTC. Further studies are needed to evaluate the role of this polymorphism in the clinical behavior of MTC.